Linear polyolefins that must be processed by melt extrusion (e.g., above 60° C.), such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and isotactic polypropylene (iPP) do not exhibit extensional flow hardening, which is a critical property for film blowing, thermoforming, extrusion casting, and foaming. Commonly, a processability “modifier” such as a long chain branched polyolefin can be added in small amounts to linear polyolefins to provide extensional hardening. So called “tree branched”, “dendritic”, and “combed” polyolefin structures are known long chain branch types that can deliver extensional flow hardening when they are added into the linear polymers. Most commonly, high pressure low density polyethylene (LDPE) having a variety of long chain branches is used as a processability additive in linear polyethylenes to enhance the blown film line speed and to provide melt strength for thermoforming. However, long chain branched polyolefins have lower toughness and their addition often compromises the mechanical properties of the linear polyolefins to which they are added. In the case of LDPE, its uses have been limited to be 20 wt % or less of the overall composition but even at 5 wt % addition the impact strength of a LLDPE would drop by 50%. Due to heterogeneous branch types present in the LDPE with ineffective star branches diluting the more effective dendritic branches, a large amount, greater than 5%, of LDPE is necessary to have any processability benefits. It is desirable to use effective long chain branched polyolefins at an amount of 5% or less that can deliver extensional flow hardening but without compromising mechanical properties.
It has been found that for an effective “comb-block polyolefin” structure to provide processability enhancement, it should possess a polyolefin backbone miscible/compatible with a linear polyolefin matrix, while possessing a comb length greater than the entanglement molecular weight in order to deliver extensional flow hardening in a linear polyolefin matrix at 1 wt % addition of the comb-block polyolefin. This is not always the case with modifiers on the market. Providing such a modifier for linear polyolefins would be highly beneficial.
Likewise for hydrocarbon and polyolefins materials that are liquid at room temperature, there is a desire to employ small amounts of viscosity modifiers to thicken hydrocarbon liquids and improve fuel efficiency in the engines in which it is used. The larger coil dimensions of polyolefin copolymers in a hydrocarbon liquid (or solvent) provide excellent thickening efficiency. However, most are linear and do not shear thin until very high shear rates thus minimizing their impact on reducing high-shear-rate viscosity/viscous loss and on improving fuel economy. Long chain branched viscosity modifiers are beneficial for shear thinning and for fuel economy, and there are multi-arm star polyolefin materials presently in the market place based on poly(hydrogenated isoprene-co-styrene) copolymers with hydrogenated polyisoprene having star arms of 20 to 40 centered on a cross-linked polystyrene core. These long chain branches deliver earlier shear thinning onset in a hydrocarbon base stock for lower viscosity at high shear rates and better fuel economy. However, their thickening efficiency is poor due to the coil dimensional shrinkage as a result of long chain branching and they are easily oxidized and degraded as a result of the presence of oxidation-prone polystyrene.
In accordance to Huang-Brown tie-chain theory (Huang, Y.-L., Brown, N, 29 J. POLYM. SCI., PART B. POLYM. PHYS., 129 (1991)), maintaining the crystallize size while reducing the coil dimension leads to fewer tie chains in-between the crystallites and thus lower toughness. This toughness erosion depends on the branch type. Providing a modifier with increased comb-branching lowers the coil dimension the least in comparison with increasing branching in star-type polyolefins or increasing generation of dendritics and tree type-structures. This lowered coil shrinkage with increasing long chain branch length (molecular weight) in comb polyolefins also makes them more desirable than star polyolefins as viscosity modifiers in hydrocarbon liquids (fluids) since its thickening efficiency, depending on the coil dimension, is less compromised while still delivering shear thinning and fuel economy.
Poly(propylene-b-isotactic propylene) comb-block copolymers were synthesized in U.S. Pat. No. 6,197,910 to Weng et al., but the isotactic combs along with the isotactic backbone does not lend itself to being compatible with base stocks or polyethylene blends.
Poly(ethylene/propylene-b-atactic propylene) comb-block copolymers were synthesized in WO 2014/120478 to Jiang et al., but having atactic polypropylene combs having a weight average molecular weight of less than 5,500 g/mole in the examples of that patent publication, which is too short for ideal flow hardening.
Poly(ethylene/propylene-b-isotactic propylene) comb-block copolymers were disclosed in U.S. Pat. No. 6,147,180 to Markel et al., the isotactic arms not suitable for compatibility in most compositions.
These and other problems are solved by providing atactic polypropylene comb-block polyolefins having combs (or “comb blocks”) with a weight average molecular weights greater than 8,000 g/mole, greater than the entanglement weight average molecular weight of atactic propylene which is 7,000 g/mole. The proper comb length imparts the extensional flow hardening when the comb block is used as a processability modifier in linear PE, in PP, or PE/PP blend matrix. This longer comb length also expands the solution coil dimensions allowing its use as a viscosity modifier in liquid hydrocarbons or polyolefins.